docs: def _compute_standard_dev(self, rup, imt, C):
          """
          Compute the the standard deviation in terms of magnitude
          described on p. 1866, eq. 6
          """
          sigma_mean = 0.
          if rup.mag <= 7.0:
              sigma_mean = (C['c12'] * rup.mag) + C['c13']
          elif rup.mag > 7.0:
              sigma_mean = (-0.00695 * rup.mag) + C['c14']
          return sigma_mean